1. Field of the Invention
This invention relates to micromachined silicon sensors or Micro Electro Mechanical Systems (MEMS) mass flow sensing technology that minimizes the disturbance around the sensor chip due to the connection of wires. This invention also provides the enhanced reliability that eliminates the sensor malfunction or damage due to the short or destruction of the exposed connection wires between the sensor chip and its carrier. The present invention further facilitates the automation process of the sensor module manufacture. This invention additionally reduces the cost of the sensor module manufacture with the reduction of wire binding of the sensor chip to its carrier and sealing process.
2. Description of the Related Art
MEMS mass flow sensors for gases have been limited to clean and dry gases, partly due to the design limitation of the most available products on market. Previously disclosure by Higashi et al. (Higashi, R. E. et al., Flow sensor, U.S. Pat. No. 4,501,144) teaches us a miniature flow speed device that could be used for measuring gas flow using the calorimetric, thermal mass flow measurement principle. The device is constructed with the MEMS process technology with a footprint of approximately 2×2 mm. The connection pads to the external control electronics are distributed along the edge of the chip front surface. Consequently, the wire connection between the device and the interface has to be exposed to the gas medium resulting in a volatile and fragile nature against fluids that may contain moisture, other conductive dilute mist, and particle, since these materials can lead to a shortage of the wire or even a destruction of the whole device. Further high speed flow pulsed flow may also create unpredictable damages to the connection wires or the devices as a whole. Alternatively, Mayer et al. (Mayer. F. and Lechner, M., Method and sensor for measuring a mass flow, U.S. Pat. No. 6,550,324) teach an integrated MEMS mass flow sensor chip using thermal pile sensing elements and CMOS integrated signal processing circuitry that effectively solve the problem for interface wire exposure and is cost effective. The device has a footprint about 3×6 mm. But the configuration also requires that the electronic control circuitry be effective sealed from the contact of the flow medium otherwise it would at least add large noises and other unexpected instabilities. Hence package of such a design requires the flow medium only passes through the sensing element but not the electronic portion of the MEMS chip that in return places a limit of a fluid channel size within about 2 mm in diameter. Therefore for most of the measurement concerned the flow channel packaged with the sensor chip could be only used for a bypass configuration of the complete measurement unit. This again limited the applications for fluid in a larger pipeline while adding possible pressure loss in the main flow channel in order to drive the gas medium into the bypass sensing configurations. Later improvement using a complicated segregated bypass structure by Ueda et al. (Ueda, N. and Nozoe, S., Flow rate measuring device, US Patent Application 2008/0314140) and Fujiwara et al., (Fujiwara, T.; Nozoe, S. and Ueda, N., Flow velocity measuring device, U.S. Pat. No. 7,062,963) to avoid the clogging of particles in the small bypass channels however did not change the basic package landscape of the bypass configuration, and the complicated channel design might only improves the failure rate of particle impact but the damages due to the presence of the liquid is still an unsolved issue.
In a later disclosure by Hecht et al, (Hecht. H. et al., Method for correcting the output signal of an air mass meter, U.S. Pat. No. 5,668,313) and Wang et al., (Wang, G. et al., Micromachined mass flow sensor and insertion type flow meters and manufacture methods. U.S. Pat. No. 7,536,908), the MEMS mass flow sensor is arranged on an elongated foot print of approximate 3×6 mm and 2×4 mm respectively, such that the binding pads on the MEMS chip front surface that connect with the electronic interface through wires are placed away from the sensing element and the wired interface can be sealed with package sealing materials such as silicone and epoxy. The configuration could then prevent the wire interface from damages due to presence of moisture and impact from conductive substances. Nonetheless, such a configuration shall create an unavoidable scaling hump on the MEMS chip front surface for which the bump shape is usually difficult to control, which would also be undesirable for maintaining the stability for the flow medium passing through the front MEMS chip surface. Further, the package processes of the said prior arts all require the wire binding and/or wire interface sealing process. These processes are both time consuming and might also incur additional reliability uncertainties due to the sealing materials stress release, false soldering during wire binding, as well as leakage of the sealing.
It is therefore desired to have a new MEMS mass flow sensor design such that the final MEMS chip package or assembly of the sensor shall result in a smooth surface for keeping the flow stability as well as for purpose of reducing the process steps such that to enhance the reliability and performance of MEMS flow sensor package or assembly.